Anyway, some minor guidelines, stories in red are essential Ultraman Zero viewing as these stories are mainly referenced in other material as this viewing guide is in chronological order.
An epilogue story for Ultraman Mebius. Events from this side story lead directly to the first Ultraman Zero story. All the essentials about this story are detailed in the 14th episode The Chronicles. Can be skipped if you are only here for Zero.
1. Mega Monster Battle - Ultra Galaxy Legend (Episodes 15-19)
The film introduces Ultraman Zero, Ultraman Belial, and the modern version of the Land of Light. A good jumping on point for new Ultraman fans as this movie's opening minutes gives us all you need to know about Ultraman and the ZAP Spacy crew, the main cast of the previous Ultra Series, Ultra Galaxy Mega Monster Battle. The Chronicles version of this story cuts the introduction montage scene.
2. Ultra Galaxy Legend Side Story - Ultraman Zero vs. Darkclops Zero (Episodes 1-3)
Story introduced Darkclops Zero, a robot copy of Ultraman Zero. Events of this story lead up to Ultraman Zero the Movie: Revenge of Belial.
3. Ultraman Zero the Movie: Revenge of Belial (Episodes 4-8)
Ultraman Belial returns, now calling himself Kaiser Belial. Story introduces the current Ultraman multiverse concept, Ultraman Zero's first human host, Rei, the Zero Eye, and Ultimate Force Zero.
4. Ultraman Zero Side Story: Killer the Beatstar (Episodes 20-22)
Ultimate Force Zero returns and features the return of Princess Emerana from Revenge of Belial and the return of ZAP Spacy.
5. Ultraman Saga (Episodes 9-13)
Features the debut of Ultraman Saga, a fusion Ultraman of Ultraman Zero, Ultraman Dyna, and Ultraman Cosmos. The movie also featured the second person to be Ultraman Zero's human host, Nozomu Taiga. The Chronicles version of this story also includes a deleted scene featuring the Ultra Brothers from the theatrical cut. Said scene is later seen in later references of this movie.
6. Ultra Zero Fight (Episodes 24-26)
Introduced three new Ultraman Zero forms as well as the return of Ultraman Belial after his defeat in Revenge of Belial. Events of this story lead up to Ultraman Geed. This is the end of Ultraman Zero's main story until Ultraman Geed.
- Ultraman Ginga S the Movie: Showdown! The 10 Ultra Warriors! - He trained Ultraman Ginga and Ultraman Victory. The duo later obtained the power to fuse and become Ultraman Ginga Victory.
- Ultraman X Episode 5 - He teams up with Ultraman X. X later obtains the Ultimate Zero Armor.
- Ultraman X the Movie: Here Comes! Our Ultraman - Another team up with Ultraman X, this time, he teams up with the Heisei Ultraman that Ultraman X met in his series as well as Ultraman and Ultraman Tiga.
- Ultraman Orb the Movie: Lend Me The Power of Bonds! - His first meeting with Ultraman Orb. He appeared in both the start and the after credits scene of the movie. Events of this movie lead up the 8th chapter of the Ultraman Orb Chronicle titled "Super Sky Great Violent-Beast Disastro", in which Zero and Orb team up with Ultraman X to fight Disastro, a monster that was hinted at the end of Ultraman X's movie. Sadly, this story wasn't made in movie or direct-to-video form.
- Ultra Fight Orb - The 9th chapter of the Ultraman Orb Chronicle, in this story, Ultraman Zero and his father, Ultraseven trains Ultraman Orb to fight a new threat, allowing Orb to unlock his new form, Emerium Slugger. Some of its story elements lead directly to Ultraman Geed.
7. Ultraman Geed
Stars Ultraman Geed, the biological son of Ultraman Belial. Ultraman Zero appears in this series in a mentor/older brother type role and the series also introduces us to Zero's third human host, Leito Igaguri and a new form for Zero, Ultraman Zero Beyond. This series also marks the conclusion of the Ultraman Zero vs. Ultraman Belial saga that started in Ultra Galaxy Legend 8 years ago.
8. Ultraman Geed the Movie: Connect the Wishes!
The final main Ultraman Geed story before Geed appears in later shows as a veteran Ultraman. Movie introduces Ultraman Geed's ultimate form, Ultraman Geed Ultimate Final.
- Ultraman R/B the Movie: Select! The Crystal Bond - Ultraman Geed appears in the movie, now as a veteran Ultraman who helped the new Ultras, Ultraman Rosso and Ultraman Blu. Movie also introduces Ultrawoman Grigio, the sister of Rosso and Blu, and Ultraman Treagear, the new recurring enemy Ultraman that will appear in the next entries of this viewing list.
9. Ultra Galaxy Fight: New Generation Heroes (Watch it on YouTube legally!)
Ultraman Zero protects the latest member of the New Generation Heroes Ultraman, Ultrawoman Grigio from Ultra Dark-Killer. It didn't go well. Oh, and a portion of the movie is pretty much the New Generation Heroes paying tribute on how Ultraman Zero helped them in the past. It's a pretty cool scene.
- Ultraman Taiga Episode 23 - Ultraman Zero teams up with Ultraman Taiga and his Tri-Squad in facing Imit-Ultraman Belial and Ultraman Tregear. Zero gives Taiga the Plasma Zero-let.
And there you have it, all of Ultraman Zero's TV, movie, and direct-to-video appearances thus far to prepare you for Ultraman Z, which will premiere on June 20th. All of these stories are available legally via TOKU, Mill Creek or Crunchyroll. The Ultraman Zero movies and direct-to-video stories are primarily fansubbed by Grown Ups in Spandex (GUIS) ages ago. You can easily find it in the usual torrent and illegal streaming sites.
Lastly, if you can't wait for Mill Creek's eventual Ultraman Zero release, Mega Monster Battle - Ultra Galaxy Legend's Japanese home video release has English subtitles and you can easily import it via CDJapan. That's how I got my copy of the film. That said, you can order your copy by clicking the links found below: